Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference, typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors include an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical solvent-based liquid electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in the above-mentioned U.S. Pat. No. 5,131,388.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an aluminum electrolytic capacitor is provided by the anodes, a clear strategy for increasing the energy density in the capacitor is to minimize the volume taken up by paper and cathode and maximize the number of anodes. A multiple anode stack configuration requires fewer cathodes and paper spacers than a single anode configuration and thus reduces the size of the device. A multiple anode stack consists of a number of units consisting of a cathode, a paper spacer, two or more anodes, a paper spacer and a cathode, with neighboring units sharing the cathode between them. Energy storage density can be increased by using a multiple anode stack configuration element; however, the drawback is that the equivalent series resistance, ESR, of the capacitor increases as the conduction path from cathode to anode becomes increasingly tortuous. To charge and discharge the inner anodes (furthest from the cathode) charge must flow through the outer anodes. With typical anode foil, the path through an anode is quite tortuous and results in a high ESR for a multiple anode stack configuration. By keeping the ESR low, however, the charge efficiency and DSR (delivered to stored energy ratio) of the capacitor are maximized.
The conduction path from the cathode to the inner anodes may be made less tortuous by providing pores in the outer anode foil. In this manner, charge can flow directly through the outer anodes to the inner anodes. Thus, the use of porous anode foil can combat the increase in ESR resulting from the use of a multiple anode stack configuration. U.S. Pat. No. 6,802,954 to Hemphill et al., incorporated herein by reference, describes an electrochemical drilling process for creating porous anode foil for use in multiple anode stack configuration electrolytic capacitors which produces a pore structure that is microscopic in pore diameter and spacing, allowing for increased energy density with a minimal increase in ESR of the capacitor. An etched foil is placed into an electrochemical drilling solution and a DC power supply is used to electrochemically etch the foil in the electrochemical drilling solution such that pores on the order of a few microns diameter are produced through the foil. The electrochemical drilling process creates large diameter “through” type tunnels, or pathways, in the foil that increase the electrical porosity of the foil, thereby improving charge efficiency and DSR. Aluminum Electrolytic Capacitors energy density is directly related to the surface area of the anodes generated in the electrochemical etching processes. Typical surface area increases achieved by etching can be 40 to 1 and represent 30 to 40 million tunnels/cm2. An electrochemical widening step is used to increase the tunnel diameter after etching to ensure that the formation of oxide will not close off the tunnels. Closing off of the tunnels during oxide formation will reduce capacitance and electrical porosity.
Adding Polystyrenesulfonic acid (PSSA) to the widening solution has been shown to improve the foil capacitance by protecting the foil surface form erosion and pitting, allowing the widening current to focus on the etch tunnel enlargement. However, the PSSA molecule has a tendency to combine with aluminum in the solution through flocculation that leads to inefficient use of the molecule to protect the surface during widening. Additionally after widening, the affinity of the PSSA to aluminum decreases the likelihood of efficiently removing the aluminum and PSSA from the tunnel structures.